This invention relates to improved thermoplastic molding compositions and, more particularly, to improved thermoplastic polyester and copolyester molding compositions.
High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319, and in Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection-moldable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
Recently, block copolyesters, wherein the major portion of the repeating units are poly(1,4-butylene terephthalate) blocks, have been found to have enhanced impact resistance.
It has now been discovered that if a small amount of a polyolefin or olefin-based copolymer is incorporated in the thermoplastic molding compositions, such as those described hereinbefore, the compositions exhibit vastly improved mold releasability.